Self-assembly of cerium-based metal-organic tetrahedrons for size-selectively luminescent sensing natural saccharides

Article abstract of DOI:10.1039/b915358f

New Ce-based Werner type tetrahedrons were achieved for size-selectively luminescent detection of natural carbohydrates through incorporating amide groups as both the multiple hydrogen bonding triggers and binding-signalling transductor.

Full text of DOI:10.1039/b915358f

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Self-assembly of cerium-based metal–organic tetrahedrons
for size-selectively luminescent sensing natural saccharidesw
Yang Liu, Xiao Wu, Cheng He,* Yang Jiao and Chunying Duan
Received (in Cambridge, UK) 29th July 2009, Accepted 20th October 2009
First published as an Advance Article on the web 9th November 2009
DOI: 10.1039/b915358f
New Ce-based Werner type tetrahedrons were achieved for size-
selectively luminescent detection of natural carbohydrates
through incorporating amide groups as both the multiple
hydrogen bonding triggers and binding-signalling transductor.
linear shape molecules H2L1 and H2L2. Here, two new
luminescence-active lanthanide tetrahedrons (TE1 and TE2)
were synthesized for the size-selective sensing of saccharides
(Scheme 1). The specific electronic structure of Ce3+ possibly
allows the better control of the assembly of highly ordered
architecture, through influencing the directional 5d orbitals
in the coordination modes.10 Taking into account the
environmentally sensitive character of these parity-allowed
electric-dipole 4f–5d transitions to the electronic conformation
of the ligands,11 the formation of hydrogen bonds with the
amide groups has the potential to affect the electron transitions
associated with the Ce3+ ions, leading to significant changes in
the optical properties.
The design of artificial carbohydrate sensors operating
through non-covalent interactions is a subject of intensive
current research, due to their broad utility in wide-ranging
applications from the food and cosmetic industries to medicinal
and academic arenas.1 Because of the subtle variation in the
sugar structures and the three-dimensional arrangement of
their functionalities, the frameworks of carbohydrate receptors
must be large enough to be able to fully encapsulate an
oligosaccharide nucleus. And the receptors should have
various patterns of preorganized, inward-directed H-bond
donor and/or acceptor functionality.2 In this regard, self-
assembled metal–organic molecular polyhedrons, appealing
as synthetic hosts, are efficient receptors for mimicking biological
carbohydrate recognition processes,3 especially when amide
groups, as multiple hydrogen bonding triggers used in nature
protein–carbohydrate complexes4 were incorporated.
Ligands H2L1 and H2L2 were obtained by reacting salicyl-
aldehyde with 2,6-dicarbohydrazide naphthalene and 1,10-
dicarbohydrazide 4,40-biphenyl, respectively. Evaporating a
CH3OH–DMF solution of these ligands with Ce(NO3)3ꢀ
6H2O in air for several days led to the formation of crystalline
solids of compounds TE1 and TE2, in a high yield (65% and
60%), respectively. EA and powder X-ray analysis proved the
pure phase of the bulky sample. ESI-MS spectrum of TE1
exhibited two intense peaks at m/z = 1084.84 and 1097.45
with the isotopic distribution patterns separated by 0.33 ꢁ 0.01,
demonstrating the presence of negatively charged species
[Ce4L1 –3H]3ꢂ and [Ce4L1 –3H * (H2O)2]3ꢂ, respectively.
On the other hand, difficulties in developing saccharide
sensors also arise from the fact that saccharides just contain
one kind of recognition unit (the hydroxyl functional group)
and lack a spectroscopic handle, such as a chromophore or
fluorophore, whose modulation could be harnessed in a sensing
scheme. Since fluorescent molecular sensing, which translates
molecular recognition into tangible fluorescence signals,5 pro-
vides an efficient tool for quantitatively detecting carbo-
hydrates with high precision in both solution and complex
media.6 The incorporation of luminescent active lanthanide
ions within the metal–organic polyhedrons represents a
promising approach in constructing Werner type cage-like
molecular capsules for luminescent detection of saccharides
in solution and/or in biological media.7 However, lanthanide
ions usually exhibit low stereochemical preferences and high
coordination numbers, the rational concepts of related Werner
type molecular polyhedrons are quite rare8 and require the use
of highly predisposed and spatially restricted ligands.9
6
6
Similarly, the two peaks at m/z = 852.20 and 1137.19 in the
ESI-MS spectrum of TE2, could be assignaed to negatively
charged species [Ce4L2 -4H]4ꢂ and [Ce4L2 –3H]3ꢂ
,
6
6
respectively. These results indicated the successful assembly
of Ce-based molecular tetrahedrons.
Single crystal X-ray structural analysisz confirmed the
formation of the tetrahedron in TE1, [Ce4(C26H18N4 O4)6]ꢀ
4C3H7NOꢀ6H2Oꢀ2CH3OH. The Ce4L1 tetrahedron comprised
6
In order to control the coordination of lanthanide ions and
obtain highly ordered architectures, highly predisposed NO2
tridentate chelators with amide groups were introduced into
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian, 116012, China. E-mail: hecheng@dlut.edu.cn
w Electronic supplementary information (ESI) available: Crystal data
in CIF, experimental details, magnetic and additional spectroscopic
data. CCDC 705880. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b915358f
Scheme 1 The constitute/constructional fragments of the functional
Ce-based tetrahedron Ce4L1 and Ce4L2 showing the cavities, the
6
6
windows (drawn in orange) and the positions of functionality groups.
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of four vertical metal centers that each coordinated to three
tridentate chelating groups in a coronary triangular prism
coordination geometry (Fig. 1). Each ligand positioned on
one of the six edges of the tetrahedron defined by four metal
ions and bridged two metal centers. The separation between
two metal ions was about 13.80 A and the inner volume was
estimated as 300 A3. The triangle face had an area of about
150 A2, potentially acting as a window for the guest molecules
to pass through. At room temperature, the wmT value of TE1
was 3.0 emu K molꢂ1. This value was globally in agreement
with the presence of four trivalent lanthanide ions as their
just resulted in weak spectroscopic variations (fluorescence
intensity increased about 10%). The association constants
(log Kass) were calculated as 2.95 and 2.87 for ribose and
xylose, respectively, indicating the smaller affinities of TE1 for
the smaller size pentoses referring to those of hexoses. The
addition of excess disaccharides including the sucrose, maltose
and trelahose did not cause any obvious spectroscopic
changes, suggesting the possible size-selective recognition of
the TE1 toward the hexoses over the smaller pentose and
larger disaccharides.
From a mechanistic viewpoint, the formation of donor-type
hydrogen bonds between the amide groups and the guest
molecules could favor electronic delocalization of the electro-
nic donors and lower the highest occupied molecular orbital
(HOMO) energy of the electronic donors,16 which would lead
to a further blocking of the PET processes that take place
between the amide and the Ce3+-based chelating units and to
a significant enhancement of the metal-based luminescence
signal. The insensitive nature of the UV-vis absorptions of
TE1 upon the addition of saccharides might also be an
indicator for such a PET mechanism.17
expected wmT value is 3.2 emu K mol .
ꢂ1 12 The decrease of the
wmT value was likely due to the thermal depopulation of the
ground-state sublevels (ESI Fig. S7w).13
Fig. 1 Molecular structure of TE1. One of the disordered parts of the
fragments, hydrogen atoms and solvent molecules are omitted for
clarity. The metal, oxygen, nitrogen and carbon atoms are drawn in
green, red, blue, and grey, respectively. Bond distances: Ce–O (phenol)
2.24 A, Ce–O (amide), 2.43 A and Ce–N 2.67 A on average.
Fig. 2 Fluorescence spectra of TE1 (50 mM) in DMF–acetonitrile
solution (15 : 85, v/v), upon the addition of a standard solution of
various saccharides, excited at 360 nm. Insert: Linear fitting for
log[(F ꢂ F0)/FL ꢂ F)] vs. log[G] for the corresponding titrations,
where F0, F and FL were the emission intensities at 525 nm of the free
TE1, of TE1 in the presence of hexoses having concentration of [G],
Luminescence spectrum of TE1 (50 mM) exhibited a ligand
based broad emission band extended from 450 to 600 nm, and
an intense band at about 525 nm as well as a weak band at
about 468 nm that overlapped on the broad band, when the
solution was excited at 360 nm. The energy difference between
the two narrow-shaped bands (2200 cmꢂ1) was close to
2000 cmꢂ1, in good agreement with the characteristic splitting
and of Ce4L1 in the presence of excess saccharides, respectively, and
6
[G] was the concentration of saccharides added.
2
of the two Ce3+ ground state levels F5/2 and the upper
According to the crystal structure of complex TE1 and some
related compounds,18 the possible separation between Ce ions
in the tetrahedral Ce4L26 cage of TE2 might be about 17 A
with an approximated inner volume of about 550 A3. It can be
expected that the Ce4L26 tetrahedron might hold the potential
to be selective to larger carbohydrates rather than the hexoses
with its larger window sizes and inner volume, given that the
recognition occurred within the cavity of the polyhedrons. The
luminescence titration of TE2 upon the addition of saccharides
2F7/2 components, induced by the spin-orbital interactions.14
Therefore, the intense emission band could be attributed to the
5d - 4f transition of Ce3+ from the lowest excited state 2D3/2
2
2
to the ground state F5/2 and the upper F7/2 components.
Upon the addition of hexoses, mannose or glucose, the
wavelength of the emission maximum (525 nm) of TE1 did
not change but the luminescence intensity enhanced gradually
with the increasing concentration of the guest. Upon the
addition of 20 mole equivalent hexoses, the fluorescence
intensity of TE1 increased by 60% for mannose and 40%
for glucose, respectively (Fig. 2). The Hill-plot profile15 of the
fluorescence titration curves at 525 nm demonstrated the 1 : 1
stoichiometric host–guest complexation behavior with the
association constants (log Kass) being 3.76 ꢁ 0.33 and
3.73 ꢁ 0.41 for that of mannose and glucose, respectively.
In the meantime, the addition of pentoses, ribose or xylose,
showed that the affinities of Ce4L2 for disaccharides were
6
larger than those for the smaller size monosaccharides. As
shown in Fig. 3, the addition of excess monosaccharides
including the hexoses and pentoses (1 mM) caused very little
spectral variations (fluorescence intensity enhancement lower
than 5%), whereas the addition of disaccharides caused a
significant luminescence enhancement (about 18% in average).
The Hill-plot profile of the luminescence at 480 nm also
ꢃc
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assignable to [Ce4L2 –2H * (C12H22O11)]2ꢂ demonstrated the
6
1 : 1 stoichiometric complexation behavior. The addition of all
the above mentioned mono-disaccharides did not cause any
obvious peaks corresponding to the host–guest species.
This work was supported by the National Natural Science
foundation of China (20801008 and 20871025) and the
Start-up Fund of The Dalian University of Technology.
Notes and references
z Crystal data of TE1: C170H156Ce4N28O36, M = 3727.71, monoclinic,
space group P21/n, black block, a = 24.009 (1), b = 37.450 (1),
c = 24.065(1) A, b = 92.650(2)1, V = 21614(1) A3, Z = 4, Dc =
1.146 g cmꢂ3, m(Mo-Ka) = 0.891 mmꢂ1, T = 180(2) K. 31 778 unique
reflections [Rint = 0.1333]. Final R1 [with I > 2s(I)] = 0.0849,
wR2 (all data) = 0.1991 for 2y = 471. CCDC number 705880.
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Fig. 3 Fluorescence responses of TE1 (red bars) and TE2 (blue bars)
for saccharides mentioned. Emission intensity was recorded at 525 nm
for TE1 (excited at 360 nm) or at 480 nm for TE2 (20 mM in
DMF/acetone solution’ 5 : 95, v/v, excited at 320 nm), respectively.
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Fig. 4 ESI-MS (a) of TE1 (0.1 mM) and (b) TE2 (0.1mM) in
DMF–methanol solution (containing 0.3 mM KOH) in the presence
of (a) Mannose (0.5 mM) and (b) maltose (0.5 mM), respectively. The
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7556 | Chem. Commun., 2009, 7554–7556